PROJECT SUMMARY The goal of this project is to functionalize quantitation of cerebrospinal fluid (CSF) huntingtin (HTT) protein as a Huntington disease (HD) biomarker. The first Phase I/IIa HTT lowering clinical trial for the treatment of HD recently concluded demonstrating dose-dependent reduction of mutant HTT (mtHTT) protein in the CSF of treated patients. The use of this measure was based on our previous work using a novel ultrasensitive mtHTT detection assay to demonstrate that suppression of HTT in the entire mouse CNS results in correlative reduction of CSF mtHTT, but further studies are required for meaningful use of this measure in the clinic. The therapeutic under trial is expected to mostly be active in superficial brain regions, while other strategies in development are expected to mostly be active in deeper brain regions or specific cell types. For this reason, it is necessary to know the regional and cell type contributions to CSF mtHTT in order to predict what a treatment-induced change means for the targeted tissue(s). Comparison of brain and CSF mtHTT in mice that express mtHTT everywhere except the brain region or cell type at study to those with ubiquitous mtHTT expression will be used to discern the proportional contributions to CSF mtHTT and provide a way to infer magnitude of treatment-induced changes in the brain by those in CSF. It will also be necessary to know how mtHTT enters CSF. Comparison of brain and CSF HTT in systems with and without neurodegeneration will be used to distinguish passive release of HTT as cells die and break open from active transport of HTT to CSF. Furthermore, the kinetics of brain clearance of mtHTT will be delineated by ectopic delivery of intra- or extracellular mtHTT protein directly to the brains of mice and subsequent measurement of brain, CSF, and plasma mtHTT at multiple post-delivery time points. Perturbation of HTT secretion and the glymphatic system will be used to investigate how mtHTT moves from inside cells of the brain to the CSF and eventually the blood. Concurrently, mtHTT will be quantified in longitudinal CSF and plasma samples from HD patients and controls and compared to phenotypic data to determine how biofluid mtHTT changes over typical clinical trial intervals and with progression in individuals, thus separating time- and treatment-dependent changes. This will also determine if CSF mtHTT concentration can be used to more accurately predict disease conversion and progression. We will also attempt to quantify wtHTT protein in CSF to investigate potential extracellular functions of HTT as well as enable allelic discrimination for trials of selective agents. The results of these studies will provide the necessary basis to interpret clinical quantitation of CSF HTT, determine which types of therapeutics could be effectively evaluated by this measure, and ascertain the predictive value of longitudinal CSF HTT quantitation in monitoring HD progression and selecting clinical trial participants.